chore: check for errors in formatter tests

[TomAFrench]

Description

Problem*

Resolves

Summary*

We're getting a lot of warnings in CI based on not using the errors variable. I've then added a check that it's an empty array.

https://github.com/noir-lang/noir/actions/runs/7009687704/job/19068675333#step:5:39

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [Exceptional Case] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[jfecher]

There's no point in asserting that is empty or only composed of warnings, why not rename errors to _errors?